1. Field of the Invention
The present invention relates to a substrate treatment apparatus which performs an etching treatment to etch a major surface of a substrate with an etching liquid, and to a substrate support to be used for the apparatus. Examples of the substrate to be subjected to the etching treatment include semiconductor wafers, glass substrates for liquid crystal display devices, glass substrates for plasma display devices, substrates for FED (Field Emission Display) devices, substrates for optical disks, substrates for magnetic disks, substrates for magneto-optical disks, and substrates for photo masks.
2. Description of Related Art
In semiconductor device production processes, a liquid treatment is often performed to treat a semiconductor wafer (hereinafter referred to simply as “wafer”) with a treatment liquid. An exemplary liquid treatment is an etching treatment which is performed by supplying an etching liquid to a major surface of the wafer. Examples of the etching treatment as herein defined include an etching treatment for patterning the major surface of the wafer (the wafer per se or a thin film formed on the wafer), and a cleaning treatment for removing foreign matter from the major surface of the wafer by utilizing an etching effect.
A substrate treatment apparatus for treating the major surface of the wafer with the treatment liquid is of a batch treatment type which is adapted to treat a plurality of wafers at a time, or of a single substrate treatment type which is adapted to treat a single wafer at a time. The substrate treatment apparatus of the single substrate treatment type includes, for example, a spin chuck which generally horizontally holds and rotates the wafer, a treatment liquid nozzle which supplies the treatment liquid toward the major surface of the wafer held by the spin chuck, and a nozzle movement mechanism which moves the treatment liquid nozzle above the wafer.
Where it is desired to perform the etching treatment on a device formation surface of the wafer to be formed with a device, for example, the wafer is held by the spin chuck with the device formation surface thereof facing up. Then, the etching liquid is spouted from the treatment liquid nozzle onto an upper surface of the wafer rotated by the spin chuck, while the treatment liquid nozzle is moved by the nozzle movement mechanism. As the treatment liquid nozzle is moved, a liquid application point at which the etching liquid is applied on the upper surface of the wafer is moved. The etching liquid spreads over the entire upper surface of the wafer by scanning the liquid application point between a rotation center and a peripheral edge of the wafer on the upper surface of the wafer (see, for example, Japanese Unexamined Patent Publication No. 2007-88381).
However, the etching liquid supplied to a center portion of the upper surface of the wafer receives a centrifugal force generated by the rotation of the wafer to be thereby moved radially outward on the upper surface of the wafer. Therefore, a peripheral portion of the upper surface of the wafer is subjected to an excess amount of the etching liquid which includes the etching liquid directly supplied thereto from the treatment liquid nozzle and the etching liquid moved thereto from the center portion of the upper surface. Thus, the peripheral portion of the upper surface of the wafer is etched at a higher etching rate than the center portion, resulting in uneven treatment of the upper surface of the wafer.
The inventors of the present invention have conducted studies on a wafer thinning process to be performed through the etching treatment by the substrate treatment apparatus of the single substrate treatment type. More specifically, with aback surface (a non-device formation surface formed with no device) of the wafer facing up, hydrofluoric/nitric acid (a mixture of hydrofluoric acid and nitric acid) having a higher etching power is supplied as the etching liquid to the back surface (upper surface) of the wafer. A surface wafer material of the back surface of the wafer is etched away with hydrofluoric/nitric acid, whereby the wafer is thinned.
The inventors of the present invention found that a difference in etching rate between the peripheral portion and the center portion of the upper surface of the wafer increases as the rotation speed of the wafer is increased during the etching treatment. Further, the inventors found that, where the wafer is rotated at a predetermined low rotation speed (40 to 60 rpm), the difference in etching rate between the peripheral portion and the center portion is reduced to improve the in-plane uniformity of the etching treatment.
If the wafer is rotated at such a low rotation speed, however, a smaller centrifugal force acts on the etching liquid on the upper surface of the wafer, making it difficult to spin out the etching liquid moved to the peripheral portion of the upper surface of the wafer. Therefore, the etching liquid is liable to remain on the peripheral portion of the upper surface of the wafer to form a thick etching liquid film (liquid puddle) on the peripheral portion. The thick liquid film contains a deactivated etching liquid in a higher proportion and, hence, has a lower etching power. In addition, the thick liquid film removes heat from the peripheral portion of the wafer, thereby reducing the temperature of the peripheral portion of the upper surface of the wafer. As a result, the etching rate is reduced on the peripheral portion of the upper surface of the wafer. Therefore, it is necessary to increase the etching rate on the peripheral portion of the wafer to further improve the in-plane uniformity of the etching treatment.
Particularly, where the etching treatment is performed on a hydrophobic wafer such as a silicon wafer which has lower affinity for the etching liquid, the liquid film (liquid puddle) formed on the peripheral portion of the upper surface of the wafer has a greater thickness, so that the etching rate is drastically reduced on the peripheral portion of the upper surface of the wafer.